1. Field of the Invention
The present invention relates to laser distance measuring systems and laser distance measuring methods for measuring the length of an object to be measured.
2. Description of the Related Art
Interferometers split light from a laser light source into at least two light beams that can be interfered, which are then sent over different light paths and subsequently recombined and interfered, and have found application in technologies for distance measurement.
Methods for distance measurement that utilize the interference of light waves include coincidence methods, in which the interference fringes at both ends of an object to be measured are observed to measure the distance, and counting methods, in which an interferometer is configured using a movable measurement reflecting mirror that is moved from the starting point to the end point of a distance to be measured to count the light and dark interference fringes that occur over this distance. A laser distance measuring system that uses a laser light source is one example of a counting method, and such systems are widely used for precise distance measurement.
FIG. 1 is a diagram that schematically illustrates the configuration of the most basic two-wavelength type movable interferometer (linear interferometer), which is a type of laser distance measuring system. A HeNe laser serving as a laser light source 1 emits a light beam having frequency components f1 and f2, which have slightly different frequencies due to the Zeeman effect created by a magnetic field that is applied to a discharge portion. The light beam with the components f1 and f2 is outputted from the light source and inputted into an interferometer. The two light beam components are circularly polarized light beams that have planes of polarization that are perpendicular to one another and that rotate in opposite directions. The two frequency components f1 and f2 of the light beam are both stabilized. The components of the light beam are subjected to photoelectrical conversion by a photodetector inside the laser light source 1, and a beat signal f1−f2 is output to a measurement electronics 11 as an electrical reference signal.
The light beam having the components f1 and f2 that is emitted from the laser light source 1 is split into its two frequency components by a polarizing beam splitter 3, which is a part of an interferometer IM.
The light beam f1 is projected to a reflecting surface 6 to be measured, such as a corner cube that has been attached to a moving object, is reflected by this surface, and is taken as measurement light. On the other hand, the light beam f2 is reflected by a reference mirror 8 such as a stationary corner cube, and is taken as reference light. The measurement light and the reference light are once again combined by the polarizing beam splitter 3 and are interfered with one another. When the polarizing beam splitter 3 and the measured reflecting surface 6 are moved relative to one another, the Doppler effect causes the frequency of the measurement light f1 to be changed by the amount Δf, that is, a Doppler component is added, and f1 becomes f1±Δf.
The light beams that are combined by the polarizing beam splitter 3 and interfered with one another are converted into electricity by the photodetector 10, and the measurement signal f1−f2±Δf of the deviated beat signal is obtained as the difference in the light frequencies by heterodyne detection. A measurement electronics 11 determines the value of ±Δf, which is the difference between the measurement signal f1−f2±Δf and the reference signal f1−f2 of the laser light source, and converts this value into position information. That is, the numerical difference between the displacement measurement signal and the reference signal is determined by a frequency counter of the measurement electronics 11 and this difference is multiplied by ½ the wavelength of the light beam. The resulting value is the distance that the measured reflecting surface 6 has moved with respect to the beam splitter.
Also, a single-beam interferometer may be used if due to space constraints the reflecting surface that is measured is small or if the reflecting surface is cylindrical or spherical.
One approach for achieving high-resolution with a laser distance measuring system that uses a single-beam interferometer is to adopt a single-beam two-path interferometer that passes the distance measurement light over the light path between the polarizing beam splitter 3 and the measured reflecting surface 6 twice so as to increase the Doppler effect and thereby raise resolution.
FIG. 2 shows the configuration of a single-beam two-path interferometer that passes light twice over interference light paths of an optical system to achieve high-resolution. In FIGS. 1 and 2, the laser light source 1 generates two light beams f1 and f2, which have planes of polarization that are perpendicular to one another and have slightly different frequencies, and are propagated and returned over the same optical axis from the light source, although for the sake of description they are shown as parallel but separate in the drawings. The single beam two-path interferometer is provided with the polarizing beam splitter 3, corner cubes (cube corer reflectors) 8 and 9 that oppose one another sandwiching the polarizing beam splitter 3 and the optical axis in between, a quarter wavelength plate 4 that is arranged on the optical axis on the output side of the polarizing beam splitter, and a quarter wavelength plate 7 that is arranged between the polarizing beam splitter 3 and the corner cube 8.
As shown in FIG. 2, the two light beams f1 and f2 that are generated by and output from the laser light source 1 pass through a non-polarizing beam splitter 2 and are incident on the polarizing beam splitter 3, where they are separated from one another.
The f1 light that is transmitted through the polarizing beam splitter 3 is reflected by the measured reflecting surface 6, which is attached to an object to be measured. If there is relative movement between the polarizing beam splitter 3 and the measured reflecting surface 6, then a Doppler component is added and f1 becomes f1±Δf. The light beam then returns to the polarizing beam splitter 3. Because the light beam f1±Δf passes through the quarter wavelength plate 4 twice, rotating its polarization plane by 90°, it is now reflected by the polarizing beam splitter 3 and proceeds in the direction of the corner cube 9. The f1±Δf light beam that is returned by the corner cube 9 is reflected by the polarizing beam splitter 3, once again passed through the quarter wavelength plate 4, reflected by the measured reflecting surface 6, becoming f1±2Δf, and then once again passes through the quarter wavelength plate 4 and returns to the polarizing beam splitter 3.
On the other hand, the f2 light beam serves as the reference light, and follows a light path that traverses the polarizing beam splitter 3, the quarter wavelength plate 7, the corner cube 8, the quarter wavelength plate 7, the polarizing beam splitter 3, the corner cube 9, the polarizing beam splitter 3, the quarter wavelength plate 7, the corner cube 8, the quarter wavelength plate 7, and finally the polarizing beam splitter 3. Here, the corner cube 8 is a reference reflecting mirror that has been fixed to the polarizing beam splitter 3. The measuring light beam and the reference light beam that return to the polarizing beam splitter 3 are once again combined, proceed toward the non-polarizing beam splitter 2 and half of them are reflected and are incident on the photodetector 10. The incident light beam, is converted into an electrical signal by the photodetector 10 through heterodyne detection and becomes the measurement signal f1−f2±2Δf. The value of ±2Δf, which is the difference between the measurement signal f1−f2±2Δf and the reference signal f1−f2 of the laser light source, is determined by the measurement electronics 11, which converts it into position information.
Thus, with a single-beam two-path interferometer, the measurement light travels twice back and forth between the interferometer and the measured reflector so that the Doppler component becomes ±2Δf, and therefore its resolution is double that of an ordinary single-beam interferometer.
As shown for example in FIG. 3, when using a laser distance measuring system that employs a single-beam two-path interferometer, the configuration of the system may necessitate the arrangement of a component that corrupts the polarized light, such as a beam bender 12, on the interference light path (between the polarizing beam splitter 3 and the measured reflecting surface 6), or the reflecting surface itself may corrupt the polarized light. In such cases, the problem arises that the reflected light is incompletely isolated by the polarizing beam splitter 3 and the quarter wavelength plate 4, and in addition to the normal return light (reflected light passed twice), abnormal return light (reflected light passed once or reflected light passed three times) also arrives at the photodetector 10. That is, after traveling from the laser light source 1 through the non-polarizing beam splitter 2, the polarizing beam splitter 3, the quarter wavelength plate 4, the beam bender 12, the measured reflecting surface 6, the beam bender 12, the quarter wavelength plate 4, and the polarizing beam splitter 3, in that order, a portion of the light that should be reflected toward the corner cube 9 instead is transmitted toward the non-polarizing beam splitter 2, becoming an abnormal return light f1±Δf, and arrives at the photodetector 10. Similarly, a portion of the twice-passed reflected light f1±2Δf that should be transmitted to the non-polarizing beam splitter 2 after traversing a normal route, that is, the route from the laser light source 1 through the non-polarizing beam splitter 2, the polarizing beam splitter 3, the quarter wavelength plate 4, the beam bender 12, the measured reflecting surface 6, the beam bender 12, the quarter wavelength plate 4, the polarizing beam splitter 3, the corner cube 9, the polarizing beam splitter 3, the quarter wavelength plate 4, the beam bender 12, the measured reflecting surface 6, the beam bender 12, the quarter wavelength 4, and the polarizing beam splitter 3, in that order, may instead be reflected toward the corner cube 9 and once again travel through the corner cube 9, the polarizing beam splitter 3, the quarter wavelength plate 4, the beam bender 12, the measured reflecting surface 6, the beam bender 12, the quarter wavelength plate 4, the polarizing beam splitter 3, and the non-polarizing beam splitter 2, in that order, becoming a three time-passed reflected light beam f1±3Δf, and arriving at the photodetector 10. When these abnormal return light beams f1±Δf and f1±3Δf are incident on the photodetector 10, not only do measurement errors occur but the abnormal light beams cause interference with the normal return light beam f1±2Δf, and this may make measurement itself impossible.